1. Field of the Invention
This application relates generally to nebulizers for use in analytical spectrometry such as inductively coupled mass spectrometry, as well as to fuel injector systems, inhalers, and the like. More specifically, the present invention is directed to methods for monitoring and controlling droplets and their characteristics in order to, for example, optimize spectrometer operating parameters to improve stability of an analytical signal and/or enhance the signal. (Throughout the specification the terms “droplets”, “aerosol” and “particles” may be used interchangeably.)
2. Description of the Related Art
Flame and plasma spectrometers are commonly used to analyze samples for their transferred into a liquid phase by procedures such as dissolution in proper solvents. The test solution is then converted into a mist by means of a variety of nebulizers, with the pneumatic ones being the most common. The mist is introduced into the hot source (i.e. flame or plasma) and undergoes sequential steps of desolvation, vaporization, atomization, excitation and ionization. The resulting atoms and ions may then be monitored by atomic absorption, atomic emission or mass spectrometric methods.
Among the steps cited above, desolvation is the most critical step and exerts the largest effect on the stability and the magnitude of the detected signal, the precision and the accuracy of the analytical measurement. Incomplete desolvation of the droplets or imperfect evaporation of the dried particles in the source results in local cooling in the analytical zone of the plasma or flame, leading to higher noise levels, reduced signal, and increased matrix effects.
Desolvation of the droplets is controlled by the quality of the aerosol droplets (i.e. size, velocity and spatial distribution in the source) and the source characteristics (e.g. temperature) governed by the plasma or flame operating conditions (e.g. operating power, chemical compositions of gases forming the plasma or flame, gas flow rates, height of analytical measurements).
In current spectrometers, constant operating parameters are maintained throughput the analysis from optimization of the analytical signal for a well-characterized standard solution as close as possible to the test solution in terms of composition. Therefore, changes in the test solution that require a new optimum operating set (e.g. change in the solvent composition) will result in alteration of the analytical signal. Due to the unknown nature of most sample solutions (e.g. environmental samples, biological materials), correction for the changes is exceedingly complex, requiring extensive sample preparation procedures, which is usually time and labor expensive, and may cause contamination of the samples.
Inductively coupled plasma (ICP) spectrometry is the current method of choice for elemental and isotopic analysis.1-3 Despite years of research and remarkable improvements in instrumentation, sample introduction is still the main problem of this powerful analytical tool. In pneumatic nebulization, the most popular method of solution introduction in ICP spectrometry, an aerosol is produced as a result of interactions between the liquid sample and a gas flow at the nebulizer nozzle.4-8 The nebulizer is typically coupled to a spray chamber to remove the coarse droplets prior to introduction of aerosol into the plasma. However, spray chambers suffer from low transport efficiencies, loss of volatile analyte, and increased memory requirements and transient acid effects.4,8-10 To alleviate these drawbacks, a test solution is directly injected into the ICP through devices such as the direct injection nebulizer (DIN) and the direct injection high efficiency nebulizer (DIHEN).11,12 Furthermore, direct injection devices offer reduced dead volume necessary for chromatography and capillary electrophoresis, minimizing the post-column broadening and improving the separation efficiency.13-16 
In both direct and indirect (conventional) sample introduction methods, the quality of the aerosol determines its fate inside the plasma, profoundly affecting the analytical performance. Ideally, the droplets must be small and slow moving, uniform in size and velocity, and confined to the central channel of the ICP. These properties lead to optimum conditions for efficient desolvation, vaporization, atomization, excitation, and ionization of the analyte in the plasma, resulting in the best analytical performance. Conventional pneumatic nebulizers, however, generate a polydisperse aerosol,17-24 leading to inefficient desolvation in the plasma. The presence of the incompletely desolvated droplets in the analytical zone of the plasma disturbs the steady-state signal generation in both ICP atomic emission spectroscopy (ICPAES) and ICP mass spectrometry (ICPMS), resulting in higher noise levels, reduced signal, and increased matrix effects.25-30 
The fate of the droplets in the plasma may be studied using high-speed photography or time-resolved spectroscopic techniques.25-35 The latter can provide the axial velocity of the atomic or ionic clouds around the droplets and analyte particles in the ICP. However, the cited clouds are typically much larger e.g., a few mm than the droplets e.g., a few μm and generally have the gas velocity of 20-25 m/s and cannot, thus, offer direct information on the size and velocity of the droplets inside the plasma. Recently, planar dropsizing and particle image velocimetry (PIV) have been utilized to characterize the size and velocity of the droplets and particles in glow discharge plasmas and flames.36,37 Also, a laser based imaging method has been developed to measure the size of the droplets in a thermal reactor.38 However, the source gas temperature in these studies was much lower than that of typical argon ICP, and the droplets were at least one order of magnitude larger than those encountered in ICP spectrometries. Small droplets (<30 μm) in high-temperature ICP (3000-7000 K) provide a challenging environment for the experimental study and optimization of the physical phenomena underlying this extremely sensitive and selective analytical technique.
Theoretical simulations are the only source of information about the characteristics of the droplets inside ICPs39-43. Importantly, no direct experimental method is available to verify the theoretical predictions to further develop the models.
Despite 100% transport efficiency, the current direct injection devices offer less than optimum spatial focusing of the aerosol into the central channel of the ICP, resulting in signal loss, elevated noise levels, matrix effects, and post-column broadening in hyphenated techniques. On the other hand, indirect sample introduction methods suffer from lower transport efficiencies, large dead volume, and spray-chamber induced matrix and memory effects, however, the droplets strictly travel in the axial channel due to the injector tube of the ICP torch. In both cases, the droplet velocities do not match the gas velocity in the axial channel. Indirectly introduced droplets lag behind the gas flow while directly injected ones exceed the gas velocity.
Directly introduced aerosols are highly scattered across the plasma torch as a result of their rotational behavior, indicating less than optimum sample consumption efficiency for the current direct injection devices.
A need exists for a system and method that provides novel insights on the behavior of the sample droplets inside an argon ICP through direct imaging of the droplets, from the tip of the nebulizer or injector to the normal analytical zone of the plasma.
In the system and method, Mie scattering from water droplets is used for imaging, providing remarkable insights into spatial distribution and evaporation of the droplets produced by three diverse sample introduction systems: 1) the DIHEN, 2) the large bore DIHEN (LB-DIHEN),44 and 3) a micronebulizer-spray chamber arrangement. Also, PIV and particle tracking velocimetry (PTV) are applied to further probe the velocity of the droplets before and after interaction with the plasma, respectively.